1. Field of the Invention
The invention relates to a method of making a planographic printing plate and a planographic printing plate obtained by using the method.
2. Description of the Related Art
The development of lasers in recent years has been remarkable and, in particular, with regard to solid-state lasers or semiconductor lasers capable of emitting light in the range from near-infrared to infrared, compact devices having high output are readily available. Conventionally, a variety of photosensitive compositions have been used as visible image forming materials or materials forming a recording layer of a planographic printing plate precursor. These lasers are very useful an exposure lights source when carrying out direct forming an image on the photosensitive compositions based on a digital data signal sent from a computer or the like.
A positive-working planographic printing plate precursor for an infrared laser contains as essential components an alkali-soluble binder resin, an infrared absorbing agent (hereinafter also called an “IR dye”) that absorbs infrared light and generates heat, or the like. In unexposed areas (image areas) the IR dye or the like functions as a dissolution inhibitor that substantially degrades the solubility of a binder resin in a developing solution by interacting with the binder resin. Meanwhile, in exposed areas (non-image areas), heat generated therein weakens the interaction between the IR dye or the like and the binder resin, whereby the exposed region dissolves in an alkaline developer to form a planographic printing plate.
However, in such a positive-working planographic printing plate precursor for an infrared laser, it cannot be said that there is sufficient difference between dissolution resistance of non-exposed areas (image areas) in a developer and the solubility of exposed areas (non-image areas) in the developer under various conditions of use, and there is the problem in that variations in the conditions of use result in susceptibility to over-development or under-development. Furthermore, fine scratches are formed by, for example, touching the surface during handling or the like, the surface state is thus easily changed, and when such a slight change in the surface state occurs, the unexposed areas (image areas) dissolve during development to form a scratch mark, causing deterioration of printing durability or poor inking properties.
Such problems are derived from an intrinsic difference in platemaking mechanism between the positive-working planographic printing plate precursor for an infrared laser and a positive-working planographic printing plate material processed by UV exposure. That is, the positive-working planographic printing plate precursor processed by UV exposure contains as essential components an aqueous alkaline solution-soluble binder resin and an onium salt or a quinone diazide compound. The onium salt or quinone diazide compound not only functions as a dissolution inhibitor by interaction with the binder resin in the unexposed areas (image areas) but also functions as a dissolution accelerator in the exposed areas (non-image areas) by decomposing under light and generating an acid.
In contrast, the IR dye or the like in the positive-working planographic printing plate precursor for an infrared laser functions only as a dissolution inhibitor for unexposed areas (image areas) and does not accelerate dissolution in exposed areas (non-image areas). Therefore, when a binder resin highly soluble in an alkaline developer is used in order to enhance the difference in solubility between unexposed areas and exposed areas of the positive-working planographic printing precursor for an infrared laser, there are the problems that the developer resistance of unexposed areas (image areas) upon development is instable, large development latitude cannot be obtained, and scratch resistance of the image areas is insufficient.
Meanwhile, platemaking methods by means of infrared laser exposure have been proposed in view of the various problems of positive-working planographic printing plate precursors. For example, European Patent No. 1834803 discloses a platemaking method that uses a developer containing at least two onium groups in which insufficient development latitude is improved. Japanese Patent Application Laid Open (JP-A) No. 2011-197303 discloses a platemaking method in which a planographic printing plate precursor, which has a positive-working recording layer that includes an upper layer containing a copolymer having a specific structural unit and a lower layer containing a water-insoluble and alkali-soluble resin and an infrared absorbing agent, is developed with an aqueous alkaline solution containing a betaine-based surfactant and having a predetermined pH value; and that the generation of development scum is suppressed and the developability of the planographic printing plate precursor after time has elapsed is improved.
From the viewpoint of handling a developer and the ease of waste liquid disposal, it is preferable to adjust the pH value of an alkaline developer used in a development step for platemaking from a positive-working planographic printing plate precursor to near-neutral, and various attempts have been made in this regard. For example, a platemaking method in which a planographic printing plate precursor having a positive-working image recording layer having a multilayer structure is treated with a developer having a pH of from 6 to 11 has been proposed (for example, see WO 2009/094120).